Yes, it is necessary. Think of Ghostery as filling the role of a black-list and ABP filling the role of a grey-list (and sometimes white-list).
3 karma · joined February 23, 2012
STASI
* Adblock Plus
* BetterPrivacy
* Ghostery
* NoScript
* PrivacyChoice TrackerBlock
* QuickJava
* User Agent Switcher
Yes, there is some overlap in functionality. And getting the configurations correct can be a minor pain (none of them are ready to go after install).
I have also found the Chromium version of Ghostery cannot be configured to block DoubleClick tracking. :-( So much for using Chrome-based browsers as my main browser.